To be able to use/equip a sling you have to have simple weapon proficeincy. Wizards have weapon proficeincy wizard which doesn't include slings according to the wiki links included above.
Hope that helps!
Thanks! It even says wizards don't get the sling. The smartest people can't operate the simplest of equipment! How true is that?! I know some engineers who would really hurt themselves with a sling
Still trying to figure out how a wizard can load a heavy crossbow
Could always be Life is Feudal. So you want to use a bow? Use a sling, wear leather and get beaten down for a day to learn how to use it, use a crossbow... and only THEN do you get to learn the bow! Want to improve your unarmed combat? Need to used padded armors and use a spear first!